Compressors performance curve

Compressor performance curves should strictly be plots of polytropic head H against suction flow measured in actual volumetric units (F ). However, it is common for discharge pressure, or the ratio of discharge to suction pressure, to replace polytropic head. In order to simplify the description of how possible flow controls operate, we will use the curves in this form. These approximations assume suction conditions and molecular weight remain constant. Should either change then the performance curve, drawn on this basis, will move. 

FIGURE 3.11 Comparison of compressor performance curve in an engine cycle. 

The main reason, however, that suction throttling reduces the amp load on the motor driver is due to the shape of the compressor performance curve (see Fig. 41.3). If we are operating on the flat portion of the curve, suction throttling will have the following effect ... 

The net result of these changes is to reduce N and increase P /Pj. Because of the shape of the compressor performance curve, this should result in a reduction in the amp load of the motor driver. A rough rule of thumb is that for every 10 percent decrease in N, the total number of moles compressed, the amp load on the motor driver will fall by 5 percent. 

From the pump or compressor performance curve, establish the flow rate, which will be the required contingency. 

FIG. 10-78 Approximate performance curves for a rotary positive-displacement compressor. The safety valve in discharge line or bypass must he set to operate at a safe value determined hy construction. 

Figure 4-17. Typical performance curves for a turbine-driven compressor train in a nitric acid plant.

Caution should be taken to prevent excessive acceleration. One solution is to provide for opening of the compressor discharge vent valve to increase the compressor flow and increase blower horsepower. This is effective with a centrifugal compressor, however, not with an axial compressor unless the compressor is provided with adjustable stator vanes that are reliable. This can be seen from the performance curve. The head versus flow curve for a given vane setting is extremely steep and opening the vent valve is ineffective. However, if the vanes operate fully open and the vent valve opens, the combined effect is satisfactory. The vent valve must not be oversized. 

Figure 1-3 presented a general form performance curve for each of the compressors. The centrifugal compressor exhibited a relatively flat curve compared to the other machines. Flat is defined as a relatively low head rise for a volume change. Translated to pressure terms, it means a relatively low pressure change for a given volume change. It is important to understand some of the basics that contribute to the curve shape. 

A recycle valve is needed for surge control as well as for the conditions listed above for reciprocating compressors. At constant speed the head-capacity relationship will vary in accordance with the performance curve. For a constant compressor speed ... 

The performance curve can also be shifted to match the process requirements by variable inlet guide vanes. Located at the compressor inlet, these vanes change the direction of the velocity entering the first-stage impeller. By changing the angle at which these vanes direct the flow at the impeller, the shape of the head capacity curve can be changed. 

The compressor can operate at any point on the performance curve. For the maximum value of suction pressure, the pressure rise across the machine at the surge control point must be less than the system pressure rating. If not, a relief valve should be installed,... 

Figure 12-32. Compressor cylinder performance curve for unloading conditions. (Elaborated on by this author based on information used by permission of Cooper-Cameron Corporation. All rights reserved.)...

Air compressors are required for many services. Figures 12-33A-C and 12-34 Table 12-6, and performance curves are presented to indicated the type of units that are suitable. Other styles, sizes, and types are available, and their omission does not indicate a lack of suitability. Also see Schaefer and Skrotzki. ... 

Power savings of 15% or more may result from the use of such vanes,depending on the design and final operating point on the compressor s performance curves. The inlet guide vanes increase the turndown and, thus, increase the operating range. 

Curve 2-2 results from a decrease in the gas density as might be represented by the factors listed. Note that these Curves 1-1 and 2-2 might represent the new rated curve for a particular set of inlet operating conditions. Because most processes cannot fix the gas analysis and system conditions exacdy, it is important to recognize the possible implications of changes in the suction conditions on the compressor performance. 

The Operating System. Regardless of calculated centrifugal compressor performance, the machine will operate only on or along its operating curve to fit the system of which it is a part. This is quite similar to the system performance of a centrifugal pump. Friction, other pressure drops of the system, and how friction varies with operating conditions determine machine performance. 

Figure 12-115A. Performance curves for rotary-vane compressor. (Used by permission A C Compressor Corporation.)...

This is not the complete view of these two compressor classes. Figure 3-70 gives the general performance curves for various positive displacement and dynamic compressors. The positive displacement compressors, particularly the multistage... 

Figure 3-70. General performance curve for various compressor types [23].

In the event of quick disconnect a torque imbalance occurs and the PCU shaft begins to over speed. If no control action is taken the terminal speed is a function of PCU loop friction losses and the turbo-machine performance curves. To prevent damage from the over speed condition, a bypass valve is opened to redirect flow from the high pressure compressor to the outlet of the turbine. 

Compressors are gas movers and devices where mechanical work is done on the gas resulting in an increased pressure. Centrifugal compressors are used in a variety of chemical process industries (CPI). Their performance often varies with changes in process conditions. Sometimes the performance curves supplied by the manufacturer as discharge pressure and power requirement against an inlet volumetric flow may not be valid for variations in process conditions. Lapina [1] has provided a technique of obtaining a usable performance curve that is valid for the actual process conditions. Gas compressor applications are employed in the following duties ... 

Real compression processes operate between adiabatic and isothermal compression. Actual compression processes are polytropic processes. This is because the gas being compressed is not at constant entropy as in the adiabatic process, or at constant temperature as in the isothermal processes. Generally, compressors have performance characteristics that are analogous to those of pumps. Their performance curves relate flow capacity to head. The head developed by a fluid between states 1 and 2 can be derived from the general thermodynamic equation. 

The output includes material and heat balances, exchanger heat curves with various physical properties, compressor performance calculations and any additional output desired through the use of fortran subroutines. These models were used to predict the plant sensitivity. 

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